Ashland, OR, USA
N745BW
MCDONNELL DOUGLAS HELICOPTER 600N
The commercial pilot reported that, during a cross-country helicopter flight, the main rotor blades seemed minimally out of track. After departing on the last leg of the flight, he noticed a slight "hop" as he made an ascending 180° right turn, but later stated that he had experienced worse with gusting winds and door-off operations. In straight and level flight, the blade track appeared to be no different than on the previous legs. During the descent toward the destination, he noticed that the hop became more apparent when the blades were unloaded. Maintenance personnel subsequently found a crack in one main rotor blade from the trailing edge forward to the spar at a point midspan near the beginning of the trim tab. Examination revealed a visible crack on the top skin, with an opposing crack on the bottom skin. The crack surfaces were flat and light grey with features indicative of fatigue cracking from the trailing edges of the upper and lower skins forward to an internal "C" channel. No gross mechanical damage was visible, and abrasions visible after stripping paint from the blade appeared to be too small to initiate the fatigue crack. The blade had been in service for about 2/3 of its service hour life and more than half of its cycle life at the time of discovery. The reason for crack initiation could not be determined.
HISTORY OF FLIGHTOn February 5, 2016, about 1430 Pacific daylight time, a McDonnell Douglas Helicopter (MDHI) 600N, N745BW, experienced a cracked main rotor blade at Ashland, Oregon. The commercial pilot was not injured; the helicopter sustained substantial damage to a main rotor blade. Brim Aviation was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 91. Visual instrument meteorological conditions prevailed, and no flight plan had been filed. The cross-country positioning flight departed Alturas, California, about 1340 and was destined for Ashland. The pilot reported that he was in one helicopter, while another pilot flew in another helicopter for the ferry flight to Ashland. Three intermediate stops were planned along the route of flight. All flight operations and characteristics had been normal, but he noted that the main rotor blades seemed minimally out of track. After departure from Alturas, he noticed a slight hop as he made an ascending 180° right turn out but stated that he had experienced worse with gusting winds and door off operations. In straight and level flight, blade track appeared to be no different than on the previous legs. During the descent into Ashland, he noticed that the hop had become more apparent when the blades were unloaded. He asked the trailing pilot to look at the rotor system for any abnormalities in flight, and the trail pilot indicated that they looked out of track. After landing, the pilot informed maintenance that the track and balance of both helicopters needed to be checked prior to the next operation. Maintenance personnel reported that there was a crack in one main rotor blade from the trailing edge forward to the spar at a point midspan near the beginning of the trim tab. After the operator discovered the crack, the blade was initially sent to Helicopter Technology Company for examination. The damage to the rotor blade was reported to the National Transportation Safety Board (NTSB) on March 9, 2016. TESTS AND RESEARCHThe damaged blade was examined by the NTSB Material's Laboratory. The blade had a time in service (TIS) of 2,013.4 hours, 11,065 torque events, and a retirement index number (RIN) of 587,640. The blade's published service life is 3,200 hours or 1 million RIN. Visual examination revealed a visible crack on the top skin with an opposed crack on the bottom skin. The crack surfaces were flat and light grey with features indicative of fatigue cracking from the trailing edges of the upper and lower skins forward to an internal "C" channel. Examination using a scanning electron microscope showed striations and other fracture features within the fatigue crack region. The initial area of origin was in the area of the trailing edge of the upper skin, and striation orientations pointed to the vicinity of the upper corner of the skin. From the origin, the fatigue crack propagated forward in the upper skin to just past the "C" channel. At the "V" strip, the fatigue crack reinitiated at the upper aft corner of the strip and propagated forward in the upper leg of the "V" and down and forward through the lower leg of the "V." In the lower skin crack surface, additional fatigue crack propagation initiated adjacent to the lower aft corner of the "V" strip then propagated forward and aft in the skin. Two additional fatigue crack paths were discovered in the "C" channel. The complete Material Laboratory Report can be found in the public docket. The other five main rotor blades were sent to the manufacturer for examination, and no anomalies were detected.
Fatigue cracking of a main rotor blade for reasons that could not be determined based on the available information.
Source: NTSB Aviation Accident Database
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